1. Field of the Invention
The invention relates to a method of fabricating semiconductor devices, and more particularly to a method of removing a patterned multilevel resist. The invention is applicable to a method of fabricating semiconductor devices, which has a substrate reproduction process in which, prior to etching a substrate such as a silicon (Si) substrate by using as a mask a resist pattern formed by a multilevel resist method, the resist pattern on the substrate is removed and the substrate is reproduced. The invention especially relates to a method of fabricating semiconductor devices which is capable of reproducing a substrate without substantially causing damage to the substrate surface in removing a multilevel resist from the substrate.
2. Description of the Prior Art
With the reduction in the size of large-scale integrated circuits in recent years, multilevel resist methods are being developed extensively, so that very fine resist patterns can be formed with high fidelity. As the multilevel resist methods are advanced, there is a strong and growing demand for a method of reproducing a substrate in a case where patterning defects arise in a resist pattern formed by the multilevel resist method. That is, if the pattern width or shape of the finished resist pattern is shifted from a design tolerance, it is necessary that the defective resist pattern on the substrate be removed for reproducing the substrate and that a resist pattern be formed once again on the reproduced substrate. A method of fabricating semiconductor devices is therefore required which has a substrate reproduction process in which a silicon containing layer used in the multilevel resist on a substrate can be removed reliably and at the same time the exposed portion of the substrate can be prevented from being etched.
In a conventional method of patterning a resin on a substrate, as a resist, there is used a single-layer positive resist of novolak resin that is heated at a low temperature of the order of 100.degree. C. prior to exposure. The reproduction of the substrate is easily performed by dissolving and removing the single-layer resist with organic solvent such as ketones and esters. The shape of the patterned single-layer resist is observed to inspect if it is within a design tolerance, and if patterning defects occur, the single-layer resist will be removed from the substrate with organic solvent. A single-layer resist is deposited once again on that substrate and patterned. If a single-layer resist is patterned within a design tolerance, it will be used as a mask for etching in the next etching process.
However, the above described conventional substrate reproduction method using organic solvent cannot be applied to the resist pattern formed by the multilevel resist method This is because the novolak resin of the underlying resist is heated at a high temperature on the order of 200.degree. C. and becomes insoluble in organic solvent in order to avoid intermixing with the upper layer to be coated on the underlying resist. For example, for a trilevel resist (a multilevel resist consisting of three layers), the underlying resist of novolak resin is heated at the temperature of the order of 200.degree. C., and the intervening layer containing silicon such as SOG (spin on glass) is coated on the underlying resist and heated. The top layer resist of novolak resin, which is coated on the intervening layer, is patterned by exposure and development, the intervening layer is patterned with the top layer pattern as a mask by RIE (reactive ion etching), and finally the underlying resist is etched by O.sub.2 RIE. For a two-level resist (a multilevel resist consisting of two layers), the underlying resist of novolak resin is heated at a temperature on the order of 200.degree. C., and the top layer resist having silicon such as polymethylsilsesquioxane (PMSS) (an electron beam negative resist), which is coated on the underlying resist, is patterned by exposure and development. Finally, the underlying resist is etched by O.sub.2 RIE.
If there are defects in the multilevel resist pattern, the reproduction of the substrate must be performed. In that case, in order to remove the defective multilevel resist pattern, which contains the underlying layer insoluble in organic solvent, a method similar to a resist ashing process is applied. Since the silicon containing layer must be also removed, an etching process using a plasma containing oxygen and a chemical substance having halogen as an element (for example CF.sub.4) or an etching process consisting of sequential stages using a plasma containing the chemical substance having halogen and using O.sub.2 plasma is adopted. However, when the surface of the substrate comprises silicon oxide, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), silicon nitride, polycrystal silicon, monocrystal silicon, tungsten, tungsten silicide or the like, the substrate is also etched during the removal of the multilevel resist because the plasma containing halogen etched the surface of the substrate. This often causes serious troubles in device fabrication on the reproduced substrate.
It is therefore an object of the present invention to provide a novel method of fabricating a semiconductor device which is capable of reproducing a substrate by removing a multilevel resist without etching the underlying substrate.